Required Safety Valve Flow for a Centrifugal Compressor

[Chance17]

I have been struggling to find a generally accepted standard for sizing a centrifugal compressor discharge safety valve.
The basic scenario is "blocked discharge".
The only thing I have found written is in the Air Compressor Industry (The minimum flow at Surge & 105% speed).
Most folks casually cite maximum flow at stonewall.
But in my thinking, this is a double contingency -- blocked discharge and rise in suction pressure.
At semi-normal suction conditions stonewall does not generate enough pressure to activate the safety valve.
Besides, why would a compressor stonewall when the discharge is blocked.
Any comments would be appreciated.

 

[TD2K]

You need to look at your system. If the suction pressure is maintained independently of the compressor then I'd tend to agree that maximum suction pressure and blocked discharge is double contingency IF the high suction pressure is truly due to an independent failure. If the suction pressure can vary depending on the plant's operation, I'd be a lot less likely to take credit for it as a double jeopardy.

If the blocked discharge allows the suction pressure to rise, then you have to consider it.

If I've sizing/evaluating a relief valve I'm trying to prevent something from being overpressured so sizing for stonewall doesn't seem to make any sense to me, back towards the surge line would make more sense to me. The relief valve could also be sized I guess to maintain a minimum flow through the unit but unless the suction contitions are sufficient stable, coming up with a set point could be tricky.

 

[MortenA]

APi 521 scetin 5.5 sattes that the capacity shall be evaluated at relieving conditions - not operating conditions.

This sets the outlet pressure.

For the inlet pressure you will have to look upstream. If this is the first compressor then you must use inlet PAHH i would think.

With regards to speed: Is it a VFD? If yes then i think you should use the max. allowed speed that would give the highest flow rate (not nescessarely max speed since it could be limited by stonewall).

Best regards

Morten

 

[CJKruger]

Yitbos, usually you do not have a relief valve for this case (block in centrifugal compressor). Just like pumps, the preferred option is to design the downstream equipment higher than the maximum discharge pressure.

 

[Chance17]

Thank You TD2K Thank You MortenA Thankyou CJKruger
for your comments